1. Field of the Invention
The present invention relates to a wiring board connection method which electrically connects a wiring board and a wiring board, and a wiring board used in the method.
2. Related Art of the Invention
In flip-chip mounting where electronic components are mounted on a substrate, bumps are formed on wiring terminals. In recent years, as a technique for forming bumps on wiring terminals, a method in which bumps are formed by making conductive particles (for example, solder powders) self-assemble on wiring terminals, or a method in which flip-chip mounting is conducted by making conductive particles self-assemble between electrodes of a wiring board and a semiconductor chip to form a connection body between the electrodes has been proposed (for example, see International Publication No. WO2006/103949), other than conventional techniques such as solder pasting and super soldering.
FIGS. 16(A) to 16(D) and FIGS. 17(A) to 17(D) are diagrams for describing a bump formation technique in which conductive particles are made to self-assemble.
First, as shown in FIG. 16(A), a resin 114 containing solder powders 116 and an air bubble generating agent (not shown) is supplied on a substrate 31 having a plurality of pad electrodes 32. Next, as shown in FIG. 16(B), a flat plate 140 is arranged on a surface of the resin 114.
Heating the resin 114 in this state, as shown in FIG. 16(C), air bubbles 30 are generated from the air bubble generating agent contained in the resin 114. Then, as shown in FIG. 16(D), the resin 114 is forced out from the area where the air bubbles exist, as a result of the generated air bubbles 30 growing.
The forced-out resin 114, as shown in FIG. 17(A), is self-gathered in the shape of columns on its interfaces with the pad electrodes 32 of the substrate 31 and on its interface with the flat plate 140. A part of the resin 114 that exists at the edge of the substrate 31 will be forced out from the outer edge of the substrate 31 (the drawing omitted).
Then, further heating the resin 114, as shown in FIG. 17(B), the solder powders 116 contained in the resin 114 are molten, and solder powders 116 contained in the resin 114 self-gathered on each pad electrode 32 are melt-bonded together.
Since the pad electrodes 32 have a high wettability for the melt-bonded solder powders 116, as shown in FIG. 17(C), a bump 19 consisting of the molten solder powders is formed on each pad electrode 32. Finally, as shown in FIG. 17(D), the resin 114 and the flat plate 140 are removed, thereby obtaining the substrate 31 with the bumps 19 formed on the pad electrodes 32. In the aforementioned process, the amount of the supplied resin 114 is shown with exaggeration, and in reality, an amount of resin 114 suitable for self-gathering on the pad electrodes 32 and determined by considering errors is supplied.
The characteristics of this method lie in heating the resin 114 supplied in a gap between the substrate 31 and the flat plate 140 to generate the air bubbles 30 from the air bubble generating agent, and as a result of the air bubbles 30 growing, the resin 114 being forced out of the area where the air bubbles exist, thereby making the resin 114 self-gather between the pad electrodes 32 of the substrate 31 and the flat plate 140, with the solder powders 116 kept contained in the resin 114.
The phenomenon that the resin 114 self-gathers on the pad electrodes 32 can be considered as occurring in a mechanism shown in FIGS. 18(A) and 18(B).
FIG. 18(A) is a diagram indicating the state in which the resin 114 is forced out onto a pad electrode 32 of the substrate 31 by the grown air bubbles (not shown). Since the resin 114 that is in contact with the pad electrode 32 has a force Fs corresponding to an interfacial tension (what is called a force resulting from the resin wet spreading) at the interface, which is larger than stress Fη generated from the viscosity η of the resin, the resin 114 spreads over the entire surface of the pad electrode 32, and finally, the column-shaped resin with the end of the pad electrode 32 as its boundary is formed between the pad electrode 32 and the flat plate 140.
Although stress Fb generated due to the growth (or movement) of the bubbles 30 is applied to the column-shaped resin 114 formed as a result of self-gathering on the pad electrode 32, as shown in FIG. 18(B), the shape of the resin 114 can be maintained by the effect of the stress Fη generated due to the viscosity η of the resin 114, and the resin 114 will not disappear once it has self-gathered.
Here, whether or not the self-gathered resin 114 can maintain a certain shape also depends on the area S of the pad electrode 32, the length L of the gap between the pad electrode 32 and the flat plate 140, and the viscosity η of the resin 114 in addition to the aforementioned force Fs corresponding to interfacial tension Fs. Where the measure for maintaining the resin 114 in a certain shape is T, it is possible to consider the following relationship to be established qualitatively. [Formula 1]T=K·(S/L)·η·Fs (K is a constant.)
As described above, in this method, the resin 114 is formed on the pad electrode 32 in a self-adjusting manner using self-gathering of the resin 114 due to its interfacial tension, but since the pad electrode 32 formed on the substrate 31 surface is formed in the shape of a protrusion, that self-gathering due to the interfacial tension can be considered to be one using a phenomenon occurring in the gap between the flat plate 140 and the pad electrode 32, which is narrower than the gap between the substrate 31 and the flat plate 140, from among the gap formed between the substrate 31 and the flat plate 140.
Using the aforementioned method, it is possible to make the solder powders dispersed in the resin 114 efficiently self-assemble on the electrode, and also to form bumps with an excellent uniformity and a high productivity. Furthermore, since it is possible to make the solder powders dispersed in the resin impartially self-assemble on a plurality of electrodes on the substrate supplied with the resin, the aforementioned method is useful especially when forming bumps at a time on all of the electrodes on the wiring board supplied with the resin.
The aforementioned technique of making the solder powders self-assemble on the electrodes by making the resin self-gather can be used not only for bump formation, but also for other usages.
As one of the other usages, the present inventors have conceived the use of that technique for connecting substrates.
In particular, for the internal wiring of electronic devices such as mobile phones and digital cameras, thin and foldable flexible print circuit boards (hereinafter, referred to as “FPC(s)”) are often used. In recent years, with the downsizing of mobile devices and the increase of movable units, the use proportion of the FPCs has been increasing. When an FPC is connected to a hard substrate used for a main board, it is common to use a connector for that connection, which provides a great advantage in that the FPC can be detached and attached repeatedly. Even though there is no need for detachment and attachment, there is an advantage in achieving easy substrate-to-substrate connection. However, the three-dimensional space occupied by the connector may hinder the provision of downsized and thinned devices. In addition, the predominant minimum pitch for the existing connectors is 0.3 mm, and it is difficult to perform electrode terminal connection with a pitch narrower than that predominant minimum pitch.
Meanwhile, a rigid-flex circuit board in which a hard substrate and an FPC are completely integrated exists. Although a rigid-flex circuit board has an advantage in not requiring a connection unit at its outer periphery because the FPC is interposed between internal layers of the hard substrate, a long manufacturing process is required and especially, where a combination of hard substrates with different numbers of layers is used, the process becomes complicated.
In these circumstances, recently, a wiring board having a structure equivalent to that of a rigid-flex circuit board can be manufactured by connecting different hard substrates via an FPC. This makes it possible to simplify the process compared to that for a rigid-flex circuit board, and provides less limitation of the outer shape and structure of the wiring board.
Accordingly, the use of the aforementioned technique can be considered to be effective for connecting wiring boards each having such narrow-pitch electrode terminals.
Meanwhile, the present inventors also discovered the following phenomenon when considering a method which connects wiring boards applying the aforementioned method. The phenomenon will be described below.
FIG. 19 shows a wiring board used for considering that connection. On a wiring board 31a, connection terminals 34a are formed in an area indicated by an arrow in the Figure by providing a plurality of strip-shaped wiring lines 33a. The width of each wiring line 33a is 0.05 mm, and the space 35a between adjacent wiring lines is 0.05 mm, and thus the wiring rule is one for a pitch of 0.1 mm. On a center portion of each connection terminal 34a on the wiring board 31a shown in FIG. 19, a proper amount of resin 114 containing solder powders and an air bubble generating agent (not shown) is applied.
Next, FIG. 20(A) shows a state in which another wiring board 31b is superposed on the wiring board 31a. FIG. 20(B) is a cross-sectional view of FIG. 20(A) taken along a straight line A-A. On the wiring board 31b, wiring lines 33b are arranged with the same dimensions as those of the wiring board 31a, and the respective connection terminals 34a and connection terminals 34b face each other, and overlap each other. Heating the applied resin 114 in this state, it can be expected to connect the wiring board 31a and the wiring board 31b as a result of the solder powders self-assembling in the area where the connection terminals 34a and the connection terminals 34b overlapping each other and then being molten and solidified.
However, when the heating is conducted in reality, as shown in FIG. 21(A), the resin 114 and the solder powders largely have moved up to the outside of the area where the connection terminals 34a and 34b overlap each other. Especially, the movement of the resin 114 and the solder powders to the spaces 35a and spaces 35b adjacent to the connection terminals 34a and 34b is significant.
FIG. 22 shows a state in which the moved and assembled solder powders have been molted and solidified. In an observation using an optical microscope, as a result of the moved solder powders being molten, regions 16a short-circuited with their respective neighboring connection terminals or regions 16b where the solder powders are assembled at the wiring lines outside the connection area although they have not yet caused short-circuiting have been observed. Furthermore, when performing an X-ray fluoroscopic observation, regions 16c having a shortage of solder and unconnected regions 16d have been observed in the connection terminals, and not all of the solder powders have assembled in the area where the connection terminals 34a and 34b overlap each other.
As described above, it has turned out that the aforementioned problems should be solved in order to connect wiring boards each having microscopic strip-shaped connection terminals by making conductive particles such as solder powders self-assemble on their electrodes.